: [Any level with room for mondo battle]
: Set it to spawn marines when AR Fires, Chiefs when Pistol Fires.
: Set it to spawn Elites when Plasma Rifle Fires, Jackals when Plasma Pistol
: Fires.
: Set it so when needler fires, grunts spawn.
: Set it when Plasma Rifle Fires, Ghosts also spawn.

: voila! WW3! =) Enjoy.

The only problem is, characters "spawned" from a gun don't have any AI. They just sit/stand there, not doing anything. It's a shame.

-Disclaimer- I might be wrong. If i am, don't come after me with pitchforks.